Protective circuit for a hacker type DC converter

ABSTRACT

A hacker type D.C. converter circuit having protective circuit features. The converter circuit includes a supply voltage source, and a series circuit including a first coil, a first diode and a first capacitor. The series circuit is connected to the supply voltage source. In addition, there is included a controllable switching arrangement, a connection junction between the first coil and first diode, and a further diode coupling across the switching arrangement. The first capacitor includes a first terminal remote from said further diode and a second terminal proximate the further diode. The connection junction is connected to the first terminal of the first capacitor remote from the further diode across the controllable switching arrangement which periodically alternates between open and closed states. The series between the connection junction and the first diode further includes an inductor and a second coil which is inductively connected to the first coil. A second diode is connected in a parallel circuit from the connection junction mentioned above and across the first diode of the first capacitor.

FIELD OF INVENTION

The invention relates to a protective circuit for a hacker type D.C.converter in which a series circuit including a coil, a diode and acapacitor is connected to the poles of a supply voltage source, and inwhich a connection point between the coil and the diode is connected tothe end of the capacitor facing away from the diode via a controllableswitching element which is adapted for periodically changing betweenopen and closed states.

BACKGROUND

A converter of the kind mention above is illustrated, for example, inGerman Pat. No. DE 2,644,715 in FIG. 25. Large power losses occur insuch a converter, particularly when converting high voltages and at highpower take-off. The losses occur particularly in the diode, throughwhich a reverse current occurs each time the switching element closes.Losses in the double also occur when it is in on-state condition in theforward direction. These so-called on-state losses dominate atcomparatively low switching frequencies for example at the supplyfrequency (50-60 Hz). On the other hand, the switching-off losses of thediode dominate when hacking at high frequency, e.g. 20 kHz, particularlyin conjunction with high voltages, e.g. 400 volts, and high currentstrengths. Apart from the disadvantages which the losses intrinsicallyare, they also result, together with the heavy voltage and currentvariations occurring in the converter, in that the diode and switch aresubjected to large stresses and are easily damaged. The currentvariations also give rise to radio interference, which can disturb theapparatus in which the converter is included and other apparatus aswell.

The known converter according to the above has been provided with aprotective circuit for decreasing the power losses and high voltages atthe switch. However, it has been found difficult to get a circuit ofthis kind to function satisfactorily. Furthermore, the losses areincreased due to energy that has been preciously stored in coilscapacitors, being subsequently consumed in resistors.

SUMMARY OF INVENTION

An object of the present invention is to provide a protective circuitfor D.C. converter of the kind mentioned above, with the aid of whichpower losses as well as rapid voltage and current variations can bereduced considerably. This is achieved in principle by two suitablyselected inductive elements being inserted in series with the diode, andin that a second diode is connected in parallel with these inductiveelements and the diode.

To achieve the above and other objects of the invention, there isprovided a hacker type D.C. converter circuit having protective circuitfeatures. The converter circuit comprises a supply voltage source, and aseries circuit including a first coil, a first diode and a firstcapacitor. The series circuit is connected to the supply voltage source.

In addition, there is included a controllable switching arrangement, aconnection junction between the first coil and first diode, and afurther diode coupling across the switching arrangement.

The first capacitor includes a first terminal remote from said furtherdiode and a second terminal proximate the further diode. The connectionjunction is connected to the first terminal of the first capacitorremote from the further diode across the controllable switchingarrangement which periodically alternates between open and closedstates.

The series between the connection junction and the first diode furtherincludes an inductor and a second coil which is inductively connected tothe first coil. A second diode is connected in a parallel circuit fromthe connection junction mentioned above and across the first diode ofthe first capacitor.

Other features of the invention include that a resistor is connected inthe above-mentioned parallel circuit in series with the second diode andthat the first diode has a lower threshhold voltage than the seconddiode. Another feature is that the first capacitor is connected inparallel with a second capacitor which from an impedance aspect is ashorter distance to the second diode than does the first capacitor. Inaccordance with a still further feature, there is provided a thirdcapacitor, a resistor and a third diode, the third capacitor beingconnected in series with the latter said resistor across which the thirddiode is connected in parallel. The switching arrangement is in parallelwith the serially-connected third capacitor and resistor.

Other features will be found in the Detailed Description which followshereinbelow.

BRIEF DESCRIPTION OF DRAWING

The invention will next be described in detail with reference to theaccompanying drawing in which:

FIG. 1 illustrates a known D.C. voltage converter; and

FIG. 2 illustrates a D.C. converter with a protective circuit providedin accordance with the invention.

DETAILED DESCRIPTION

A known D.C. converter of the hacker type is illustrated in FIG. 1. Inthis known converter, a coil L1, a diode D1 and a capacitor C1 areconnected in series to the poles of a supply voltage source U1. A load Lis connected in parallel with the capacitor. A connection point pbetween the coil and diode is connected to the negative pole of thevoltage source via a controllable switching element T in the form of atransistor. If the transistor is controlled to be alternatingly on-stateand off-state, the voltage U2 across the capacitor and load will behigher than the supply voltage U1, and then the converter functions as avoltage increasing circuit.

Since the capacitor C1 will be positively charged on its side facingtowards the diode D1, the diode will be biassed in the reverse directioneach time the transistor T becomes on-state. Before the diode blocks inthe reverse direction, a charge stored during its on-state in theforward direction gives rise to a reverse current during the dioderecovery time. The reverse current involves a power loss, which will beconsiderable for hacking at high frequency, particularly in convertinghigh voltages and at high power take-offs. As will be seen from theabove, the power losses and the heavy voltage and current variationscause the diode and transistor to be subjected to large stresses and theoccurrence of radio interference.

An embodiment of a D.C. converter with a protective circuit provided inaccordance with the invention is illustrated in FIG. 2 in which the coilL1, diode D1, capacitor C1 and load L have their counterparts in FIG. 1.The switching element T may comprise a transistor, as with the knownconverter according to FIG. 1. Two further coils L2 and L3, of which thecoil L2 is inductively connected to the coil L1, are in series with thecoil L1 and the diode D1 inserted between the point p and the diodeanode. The coil L3 can either consist of an optionally selectedinductive element with suitable inductance, or it may be achieved ineffect with the aid of the leakage field from the coil L2. All threecoils may be provided by a single coil in practice.

Since the current through a coil has the tendency of only changingcomparatively slowly, there is obtained, due to the coil L3, aconsiderably weaker and shorter duration reverse current through thediode D1 when the switch T is switched on than in conventionalconverters of this kind. This gives a lower power loss in the diode andsimultaneously a milder current shock through the switch.

A diode denoted D2 is in series with a resistor R2 and connects thepoint p to the capacitor C1. The task of the diode is to be a path forthe current from the coil L1 directly after the switch TT is switchedoff. At the beginning of each switch-off period namely the coil L3namely prevents the current from the coil L1 from going through thediode D1. The current path R2-D2 also limits the growth of the voltageacross the switch on switching off, as this voltage could otherwisedamage the switch. To limit this voltage, the switch is also connectedin parallel, in a manner known per se, with a circuit consisting of acapacitor C3 in series with a resistor R3, accross which a diode D3 isconnected.

When the switch T is switched off, the current flows first for a shorttime through the circuit connected in parallel with the switch, thecapacitor C3 then being charged. When the current through this circuitdecreases, it increases through the resistor R2 and diode D2. Finally,the current increases through the diode D1, which is accelerated by ane.m.f. occurring in the coil L2 and generated due to the inductivecoupling between the coils L1 and L2. By suitable mutual adjustment ofthe components, the current is transferred entirely from the branchR2-D2 to the branch L2-L3-D1, whereafter it is also retained in thisbranch. This depends above all on that the e.m.f. in the coil L2, atleast to a certain extent, can be caused to occur during the whole timethe switch is off. The transfer and retention is further facilitated bythe resistance of the resistor R2, and if the diode D1 has a lowerthreshold voltage than the diode D2. The resistor R2 may possibly beexcluded. However, it is essential that the current through the diode D2ceases before the switch T is closed again, since a reversed currentwould otherwise occur in this diode.

The capacitor C1 suitably is an electrolyte capacitor with relativelyhigh capacitance. A smaller capacitor C2 is connected in parallel withit, but closer to the diode D2 than where the capacitor C1 is situated,from the point of view of impedance. The task of this smaller capacitoris to be a low-impedance path for high-frequency currents from thebranch R2-D2, although it is conceivable to exclude this capacitor.

When the switch T is switched on, there may be a higher voltage acrossthe diode D1 than across the capacitor C1, due to that the potential atthe diode anode can be lower than the potential at the negative pole ofthe supply voltage source U1. This high voltage is reduced by an RC linkconnected across the diode and consisting of a resistor R4 in serieswith a capacitor C4. The voltage is also reduced by an attenuationcircuit between the diode D1 anode and the negative supply pole. Thiscircuit comprises a diode D4 in series with a capacitor C5, which isconnected in parallel with a resistor R5 in series with a Zener diodeD5. When the potential at the diode D1 anode becomes lower than thepotential at the negative pole of the supply voltage source, the diodeD4 assumes its on-state and the capacitor C5 is charged. The voltageacross the diode D1 is thus substantially limited to the sum of thevoltages across the capacitors C1 and C5. The voltage across thecapacitor C5 is determined by the Zener voltage of the diode D5, andwith suitable values for this and the resistance of the resistor R5 thelosses in these components can be limited to modest values.

By way of summary it may be said that the losses and stresses in thediode D1 and switching element T may be substantially limited by thecircuits shown, while high-frequency interference fields are heavilyreduced.

Examples of component values for a functioning circuit will be seen fromthe list below.

L1: 1 mH R2: 0.1 ohms R5: 10 ohms C3: 1 nF

L2: 10 μH R3: 100 ohms C1: 5,000 μF C4: 1 nF

L3: 10 μH R4: 100 ohms C2: 1 μF C5: 0.1 μF

D5 Zener voltage: 50 V

We claim:
 1. A hacker type D.C. converter circuit comprising a supplyvoltage source, a series circuit including a first coil (L1), a firstdiode (D1) and a first capacitor (C1), said series circuit beingconnected to the supply voltage source (U1), controllable switchingmeans, a connection junction between said first coil and first diode, afurther diode (D3) and capacitor (C3) coupled across said switchingmeans, said first capacitor including a first terminal remote from saidfurther diode and a second terminal proximate said further diode and animpedance means, said connection junction (p) between the first coil(L1) and the first diode (D1) being connected to the first terminal ofthe first capacitor (C1) remote from the further diode through saidimpedance means across said controllable switching means (T), whichperiodically alternates between open and closed states, the seriescircuit between said connection junction (p) and the first diode (D1)further including an inductive element (L3) and a second coil (L2) whichis inductively connected to the first coil (L1), and a second diode (D2)connected in a parallel circuit from the connection junction (p) andacross the first diode (D1) to the first capacitor.
 2. A circuit asclaimed in claim 1, comprising a resistor (R2) connected in saidparallel circuit in series with the second diode (D2).
 3. A circuit asclaimed in claim 1 or 2, wherein the diode (D1) has a lower thresholdvoltage than said second diode (D2).
 4. A circuit as claimed in claim 1,comprising a second capacitor (C2), the first capacitor (C1) beingconnected in parallel with said second capacitor (C2).
 5. A circuit asclaimed in claim 4, comprising a third capacitor, a resistor (R3), and athird diode (D3), said third capacitor (C3) being connected in serieswith said resistor (R3) across which said third diode (D3) is connectedin parallel, said switching means (T) being in parallel with theserially connected third capacitor and resistor.
 6. A circuit as claimedin claim 5, comprising an RC link comprising a fourth capacitor (C4) anda further resistor (R4) in series with ssaid fourth capacitor (C4), saidRC link being connected in parallel with the first diode (D1).
 7. Acircuit as claimed in claim 6, including a further circuit comprising afourth diode (D4) in series with two branches connected in parallel, ofwhich one branch is a fifth capacitor (C5) and the other branch anadditional resistor (R5) in series with a Zener diode (D5), said furthercircuit being connected between the first diode (D1) and the firstcapacitor (C1).